HTTP/3 frames — RFC 9114 §7

Every HTTP/3 frame has the common shape:

HTTP/3 Frame Format {
    Type (i),              ← QUIC varint
    Length (i),            ← QUIC varint: payload byte-count
    Frame Payload (..),
}

Frames are carried on:

• Control stream — server-initiated uni stream 0x03; SETTINGS, CANCEL_PUSH, GOAWAY, MAX_PUSH_ID.
• Request / response streams — bidi streams 0x00–0xFFFFFFFC; DATA, HEADERS, PUSH_PROMISE.
• Push stream — server-initiated uni stream 0x01; HEADERS, DATA.

Both Type and Length are QUIC varints (§16). Unknown frame types on request or control streams MUST be ignored (§7.2.8). GREASE frame types 0x1F × N + 0x21 from RFC 8701 exercise that discipline.

Frame catalogue

Type	Name	Control	Request	Semantics
0x00	DATA	—	✓	Body byte stream
0x01	HEADERS	—	✓	QPACK-encoded field section
0x03	CANCEL_PUSH	✓	—	Reject a server push (push_id varint)
0x04	SETTINGS	✓	—	Parameter list (id, value) varint pairs
0x05	PUSH_PROMISE	—	✓	Server push announce (push_id + headers)
0x07	GOAWAY	✓	—	Graceful shutdown (stream_id or push_id)
0x0D	MAX_PUSH_ID	✓	—	Raise push budget (max_push_id)
any	ReservedFrame	both	both	Pass-through, ignore .8

SETTINGS frame body (§7.2.4)

SETTINGS Frame {
    Type (i) = 0x04,
    Length (i),
    Settings Entries (..) {
        Identifier (i),
        Value (i),
    }
}

Registered identifiers:

ID	Name	Default	RFC
0x01	QPACK_MAX_TABLE_CAPACITY	0	RFC 9204 §5
0x06	MAX_FIELD_SECTION_SIZE	u64.MAX	RFC 9114 §7.2.4.1
0x07	QPACK_BLOCKED_STREAMS	0	RFC 9204 §5
0x08	ENABLE_CONNECT_PROTOCOL	0 (off)	RFC 9220
0x33	H3_DATAGRAM	0 (off)	RFC 9297

core.net.h3.settings.H3Settings.defaults() matches the spec defaults. from_entries / to_entries round-trips the wire form.

Byte-exact SETTINGS with qpack_max_table_capacity=4, max_field_section_size=8:

[0x04]        ← SETTINGS type
[0x04]        ← length = 4
[0x01 0x04]   ← qpack_max_table_capacity = 4
[0x06 0x08]   ← max_field_section_size = 8

See frame_encode_kat and settings_codepoints.

Frame legality (§7.2)

Frame	On control	On request	On push
DATA	✗	✓	✓
HEADERS	✗	✓	✓
CANCEL_PUSH	✓	✗	✗
SETTINGS	first only	✗	✗
PUSH_PROMISE	✗	✓ (client → server)	✗
GOAWAY	✓	✗	✗
MAX_PUSH_ID	✓	✗	✗
ReservedFrame	✓	✓	✓

The ADT carries this in two predicate methods:

impl H3Frame {
    public fn is_control_stream_legal(&self) -> Bool;
    public fn is_request_stream_legal(&self) -> Bool;
}

Illegal frame on the wrong stream triggers H3Error.FRAME_UNEXPECTED.

Connection establishment

First byte on the control stream must be the stream-type prefix (varint 0x00 for control), followed immediately by a SETTINGS frame:

[0x00]                                ← control stream type (varint)
[0x04] [len] [settings entries...]    ← SETTINGS frame
[further control frames...]

Other server-initiated uni streams:

• QPACK encoder stream — type 0x02
• QPACK decoder stream — type 0x03

Client emits its own encoder / decoder streams mirroring the roles. See QPACK.

Server push flow

Client permits push by advertising MAX_PUSH_ID; server opens a PUSH_PROMISE on a request stream referencing a pushed-resource push_id; server subsequently opens a push uni-stream carrying the response headers + body.

PUSH_PROMISE wire:

[0x05]                          ← type
[length]                        ← varint: push_id_len + encoded_headers_len
[push_id]                       ← varint
[encoded_headers ...]           ← QPACK field section

Flow control:

• CANCEL_PUSH(push_id) — either side rejects an outstanding push.
• MAX_PUSH_ID(id) — client raises the server's push allowance.

See server push.

GOAWAY

Graceful-shutdown signal on the control stream. Payload is a single varint:

• From server: the last stream_id the server will process.
• From client: the last push_id the client will accept.

Further streams above that id must be reset with H3_REQUEST_CANCELLED.

References

• core/net/h3/frame.vr
• vcs/specs/L2-standard/net/h3/frame_encode_kat.vr
• vcs/specs/L2-standard/net/h3/frame_roundtrip.vr
• vcs/specs/L2-standard/net/h3/frame_type_codepoints.vr
• vcs/specs/L2-standard/net/h3/settings_codepoints.vr
• vcs/specs/L2-standard/net/h3/frame_qpack_typecheck.vr
• vcs/specs/L2-standard/net/h3/h3_error_variants.vr